It is frequently desirable to know the volatility of a material. One measure of volatility is vapor pressure, which is defined as the pressure at which a liquid and its vapor are in equilibrium at a given temperature. Higher vapor pressure indicates greater volatility.
It is desirable to know the volatility of a material for many reasons. Among these are to predict the degree to which a material will pollute the air. The vapor pressure of a material also corresponds to the danger to workers caused by inhaling fumes of that material. The flammability of a material also corresponds to its vapor pressure as well as the ability of a material to dry, or skin over. Many products have vapor pressure specifications and determining the vapor pressure of these products is required for quality control.
Many materials do not have a significant vapor pressure at room temperature but will have a high vapor pressure at elevated temperatures. For example, roofing tars neither pollute the air nor endanger workers who work with them when they are cool and installed on a roof but the ability of these tars to pollute the air, endanger workers, or burst into flames is significant at the high temperatures at which they are maintained when they are being installed. The vapor pressure of roofing tar also corresponds to how rapidly flame spreads in a fire.
Measuring vapor pressure is a difficult and time consuming process that frequently requires a skilled analyst. Even measuring the vapor pressure of relatively volatile materials, such as gasoline, requires establishing equilibrium conditions between the liquid and the gas and then making the delicate measurements necessary to determine the vapor pressure of the material. Measuring the vapor pressure of materials having low volatility is extremely difficult because the vapor in equilibrium with the material is at such low concentration.
Known methods for measuring vapor pressure are too slow to be useful as an industrial tool or too complex or expensive to be used for frequent or repetitive measurements.